Danish director Mads Hedegaard had to invent new languages for the Stone Age epic “Stranger.”

“I have a background in documentaries [‘Cannon Arm and the Arcade Quest’], so it had to feel real to the audience. I was like: ‘We’re not going to buy it if the actors talk in Danish or English,’” he explained to Variety before the film’s world-premiere at Sweden’s Goteborg, where it is the festival’s closing film.

“There was a guy I knew; he has a PhD in Proto-Indo-European languages. But even those are 2,000 years older than those in our story. We asked him: ‘Can you come up with new ones?’ Tobias [Søborg] took an old Siberian tribal language and merged it with ancient Mayan. Then he did the same thing again, this time using dialects from the area around Turkey. We had a dictionary and a set of grammatical rules. Poor actors. They had to learn so much more than just their lines,...